A new view of the cell nucleus has emerged based on the functional dynamics of nuclear architecture. The striking structural preservation of a variety of genomic processes on the nuclear matrix provides an important approach for correlating nuclear form and function. In situ labeling coupled with three dimensional microscopy and computer imaging techniques reveal that DNA replication and transcription sites are organized into higher order units or "zones" in the cell nucleus. The dynamic interplay and "rezoning" of replication and transcription regions during the cell cycle may form the structural basis for the elaborate global coordination of replicational and transcriptional programs in the mammalian cell. As one basis for this functional organization, the genome is arranged into discrete chromosome territories in the interphase nucleus. With these previous studies as a background we propose the following specific aims: 1. Further development of multi-dimensional computer imaging approaches to 3-D microscopy. 2. Elucidate the 3-D spatio-temporal properties of DNA replication and transcription sites for genes replicating in early S-phase. 3. Study the 3-D arrangement of chromosome territories in the cell nucleus. 4. Investigate the dynamics of chromatin domains and their 3-D arrangement in living cells.